The invention is concerned with the growth of single crystal epitaxial layers of compounds represented by the formula MNO.sub.3 and solid solutions of two such compounds, where M represents an alkali metal, and N is either niobium (Nb) or tantalum (Ta). The single crystal epitaxial layers are deposited upon a single crystal substrate which also is represented by the formula MNO.sub.3.
Integrated optics is an area which has witnessed increased developmental activity because it offers the prospect of inexpensively producing efficient optical devices. Furthermore, integrated optics offers the potential of practically producing fiber optic communication systems. A key element in this emerging technology is the ability to produce thin waveguide layers of specific compositions and thicknesses in a reproducible manner. A waveguide layer is a thin layer (having a thickness on the order of the wavelength of the guided light) of a non-linear, optically active material possessing a slightly greater refractive index than its surroundings. Waveguide layers of particular interest are those suitable for use at wavelengths of 1.06 microns.
Single crystals of MNO.sub.3, in general, and more specifically LiNbO.sub.3 and LiTaO.sub.3 have generated much interest because these crystals are non-linear, optically active materials exhibiting low optical absorption (preventing large losses) at 1.06 microns, the wavelength of interest. Two methodologies, in particular, have been employed to process such bulk single crystals to achieve the resultant structure of a thin waveguide layer possessing a slightly larger refractive index than the bulk crystal.
One such technique is an out diffusion process which consists of heating a LiNbO.sub.3 or a LiTaO.sub.3 single crystal at 1100.degree. C, in vacuum, for periods from about 1 to 5 days. During this high-temperature vacuum exposure, out diffusion occurs at and near the surface of the crystal toward the bulk or body of the crystals. It has been hypothesized that the weaker bonded, smaller lithium atom will tend to diffuse more rapidly than the niobium or tantalum atoms. As a result, the region of material adjacent to the surface becomes depleted in lithium, compared to the bulk, resulting in a change in the refractive index of this thin layer. Since depletion of lithium results in a higher refractive index, a positive waveguide layer is produced. Weight gain measurements indicate, however, that lithium (hypothetically in the form of Li.sub.2 O) is not the only species removed from the thin surface layer. Thus it appears that this technique does not provide the proper control over the resultant composition of the bulk crystal. This is significant because in actual application, optical properties of both the bulk crystal and the thin waveguide layer must be carefully controlled, which involves, at a minimum, definition and control of the composition of the bulk crystal and of the thin waveguide layer.
A distinctly different method, which has been utilized to produce a thin epitaxial waveguide layer of LiNbO.sub.3 on a LiTaO.sub.3 substrate, employs melting a charge of LiNbO.sub.3 positioned upon a LiTaO.sub.3 substrate. This requires heating the substrate and overlying charge to a temperature exceeding the melting temperature of the LiNbO.sub.3, but below the melting temperature of the LiTaO.sub.3 substrate. Thereafter, the temperature is reduced at a rate of about 20.degree. C/hour, until the LiNbO.sub.3 is solidified. Typically, the temperature is raised to about 1300.degree. C, or approximately 50.degree. C above the melting temperature of LiNbO.sub.3 and about 250.degree. C below the melting temperature of the LiTaO.sub.3. This technique is referred to by those familiar in this art as expitaxy-grown-by-melting (EGM). Because of the fortuitous relationship between the melting temperatures of these two constituent compounds, EGM can be employed to produce an epitaxially grown layer of LiNbO.sub.3 single crystal on a LiTaO.sub. 3 substrate. However, this technique requires operation at significantly elevated temperatures, with all the attendant physical difficulties and drawbacks. Furthermore, subjecting the crystals to high temperature increases the potential of damaging the crystals by thermal shock, and the necessity of cooling the heterogeneous resultant structure over a large temperature differential can result in thermal expansion mismatch stresses. Also, in practical applications, it is desirable to tailor the index of refraction difference between the layer and substrate to obtain the desired waveguide effects. This can be achieved by using a solid solution of LiTaO.sub.3 -- LiNbO.sub.3 in lieu of the overlying LiNbO.sub.3 charge. However, as LiTaO.sub.3 is added to the LiNbO.sub.3, the melting temperature of the overlying charge increases upward toward the melting temperature of the LiTaO.sub.3, and indeed a distinct melting temperature no longer exists, but is replaced by a melting range. This is detrimental to the process in that higher temperatures will be required and, upon solidification of the melt, segregation will occur.